1. Field of the Invention
This invention relates to thermosetting silicone resin molding compositions. More particularly, this invention relates to silicone resin compositions adaptable to transfer molding techniques.
2. Background Information
Silicone resin molding compositions are prepared by blending a silicone resin, a filler, and a catalyst for curing the composition. Because silicone resins are generally hard and brittle at room temperature, it is necessary to heat the resin while the filler and catalyst are added and mixed to make the composition. The heated silicone resin becomes soft and the filler and catalyst can be uniformly blended to form the composition. The problem is apparent. The catalysts, for curing the resin when heated, are to be mixed into hot resin to obtain a uniform blend. A solution to this problem in the past was to dissolve the resin in a solvent to permit adequate blending of the ingredients without heating them. However, many applications are not suitable for compositions which use solvents. Also, solvent, particularly many organic solvents, are undesirable from an environmental standpoint. For those situations which require a solventless silicone resin molding composition, a catalyst with unique properties is required, namely one which will permit the silicone resin to be blended with the catalyst and filler while the silicone resin is heated, but will not cause the silicone resin to cure until it is used in the molding process.
It has been known for some time that lead salts of carboxylic acids are good catalysts for siloxane resins. This is especially true for siloxane resins which are used in solvent. In this situation, the siloxane resin can be put into a solvent at room temperature and the catalyst and filler can then be mixed with it. However, attempts to use these lead catalysts in solvent free silicone molding compounds have been unsuccessful because they cause gelation during the mixing process because of the heat.
In addition, lead catalysts are not desirable for use in transfer molding because they often gel between the pot and the mold. This causes insufficient flow and prevents satisfactory moldings from being formed. Even in those situations where partial gelling occurs at certain points of the transfer, gel particles can cause poor molded articles and non-uniform products.
From U.S. Pat. No. 3,208,961, issued Sept. 28, 1965, I found that combinations of lead monoxide or lead carbonate with either carboxylic acid or ammonium salts of carboxylic acids could be used as a catalyst for phenyl-containing silicone resin molding compositions where they could be prepared by hot mixing the resin, filler, and catalyst, still have sufficient flow for use in transfer molding, and would cure rapidly in the mold to give a hard molded article. However, lead catalysts cannot be used in some transfer molding processes. Additionally, many of the lead salts are environmentally toxic which is a disadvantage.
In the science of producing investment casting cores by the transfer molding process, using compositions containing silicone resins and oxide fillers, such as fused silica, alumina, and zirconia, as well as, pigments and processing aids, lead and other heavy metal compounds can not be used as catalysts. Lead compounds in the final fired core may react with the casting metal to cause flaws in the final casting. The silicone resins normally used in these compositions are solid at room temperature and contain predominantly reactive hydroxyl groups on silicon, however, some alkoxy groups on silicon may also be present. Condensation of these groups in the presence of a catalyst causes the composition to cure to a rigid thermoset condition.